Bonded grid-cathode electrode structure

ABSTRACT

A variety of technologies have been applied in the development of a bondedrid cathode. Erosion lithography is used for making the fine-detail grid structure, combining air erosion and lithographic techniques. To obtain openings of the order of 0.001 inch (one mil) or smaller, a nozzle with a high aspect ratio exit opening is used, and the cathode grid structure is scanned. A photo resist in which the grid pattern is developed is used over the molybdenum or tungsten grid film. The metal film is removed from the grid openings by chemical etching. The photo resist over the metal grid is used as a composite mask for removing the BN insulation in the openings by erosion with Al 2  O 3  powder from the special nozzle on the air blast gun.

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalties thereon or therefore.

This application is a continuation of application Ser. No. 149,204,filed May 12, 1980, now abandoned, which is a division of Ser. No.037,258, filed May 12, 1979, now U.S. Pat. No. 4,237,209.

BACKGROUND OF THE INVENTION

This invention relates to erosion lithography and nozzles; and inparticular to manufacture of a bonded cathode and electrode structurefor microwave triode tubes.

The grid-controlled power amplifier has long been useful for a varietyof microwave applications. The L-64 and L-67 types, developed by J. E.Beggs and his associates as a consequence of work sponsored by theUnited States Army Electronics Command, have extended the range ofperformance of such devices. These advances were attained through theuse of a closely spaced grid-cathode structure operating in thehigh-vacuum environment of a titanium-ceramic tube structure.

The construction of grid-cathode units with even closer spacing of gridand cathode and capable of high grid dissipation was continued using agrid and a heater which are rigidly bonded to the cathode by aninsulating film. Boron nitride (BN) was identified as the preferredinsulating material. Chemical vapor deposition (CVD) of BN wasdeveloped, and grid patterns with detail as small as 0.002 inch wereformed by erosion through a mask with air driven Al₂ O₃ particles. Thed-c characteristics of bonded grid tubes showed a high utilization ofemission as useful plate current, ability to withstand large positivegrid bias, and the option of a high level of current collection or awide grid-anode gap. See U.S. Pat. Nos. 3,599,031; 3,638,062; and3,694,260 by J. E. Beggs.

Several significant technical problems remained, potentially blockingthe successful development of still further improvements at highermicrowave frequencies of a bonded grid triode. One of these was lack ofa process for forming grid openings with dimensions as small as 0.001inch without either undercutting the supporting insulation or shortingout the insulating layer with metal.

The use of erosion lithography for electrical components is shown inU.S. Pat. No. 3,392,052 by J. Davis. The practice has been to usecylindrical nozzles for sand blasting or erosion lithography withpowered abrasives, as shown, for example, in U.S. Pat. No. 3,032,930 byS. B. Williams. However, in attempting to use cylindrical nozzles forfinely detailed patterns over large areas, it has been the experiencethat in spite of favorable erosion rates, the patterns would becompletely destroyed in some regions and not cut at all in others.

A method of removing photo resist in a partial pressure of a gas, whichmay be hydrogen, at about 100 degrees C. is shown in U.S. Pat. No.3,837,856. Other U.S. patents on removing resist are U.S. Pat. Nos.3,787,239; 3,582,401; 3,458,401; 3,458,312; and 3,676,219.

SUMMARY OF THE INVENTION

An object of the invention is to provide a fine pattern with openings ofthe order of 0.001 inch or less, in particular, for the grid pattern ofa bonded grid-cathode structure.

A feature of the invention is the method of erosion lithography using anozzle with a high aspect ratio and scanning the structure being workedwith respect to the nozzle. Another feature relates to the nozzle havinga high aspect ratio.

In the method of making a bonded grid-cathode unit, an insulating layeris formed on the cathode, a molybdenum or tungsten metal grid film isformed on the insulator, photo resist is formed on the grid film and thegrid pattern developed therein, the grid openings in the metal film areremoved by chemical etching, and erosion using the high aspect rationozzle while scanning the grid-cathode unit removes the insulation inthe openings down to the cathode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application partially discloses matter claimed in relatedapplications to be filled on the same day in the same package. Theothers are incorporated herein and made a part hereof as though fullyset forth.

Features relating to the high resistivity electrical insulating layersof boron nitride with a diffusion barrier of silicon nitride are coveredin an application by D. W. Oliver and C. R. Trzaskos, Ser. No. 037,257,now U.S. Pat. No. 4,150,428.

The combination getter and internal structure with heat shield iscovered in an application by D. W. Oliver and N. T. Lavoo, Ser. No.037,256, now U.S. Pat. No. 4,223,243.

DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a prior art bonded grid-cathode-heater unit for amicrowave vacuum tube;

FIG. 2 is a graph showing rates of removal for different materials witherosion guns;

FIG. 3 illustrates the velocity effect of an air erosion gun withcylindrical nozzle;

FIGS. 4 and 5 show adapter nozzles with high aspect ratio rectangularexit openings;

FIG. 6 illustrates the velocity effect of an erosion gun with arectangular nozzle exit;

FIG. 6A shows a rectangular exit nozzle shape which can be used in theabrasion process;

FIG. 7 shows the method of air erosion lithography with a high aspectratio nozzle and scanning;

FIG. 7A shows a view of a cathode-grid unit which may be subjected to anabrasion process;

FIGS. 8 and 9 are views of an air erosion gun with rectangular nozzle,suction and orifice;

FIGS. 10, 11, and 12 are diagrams of the exit openings of differentconfigurations for high aspect ratio of air erosion nozzles;

FIG. 13 is a diagram of a section of a bonded grid-cathode structure,indicating steps of formation and the functions

FIG. 14 shows a cathode blank as received from the manufacturer; and

FIG. 15 is a drawing from a photomicrograph of a grid structure made byerosion lithography.

DETAILED DESCRIPTION

FIG. 1 shows a cross section of a prior art bonded heater-cathode-gridstructure for use in the microwave power-amplifier tube disclosed inU.S. Pat. No. 3,638,062 by J. E. Beggs. It embodies a cathode disk(twin-grooved around its edge, boron nitride (BN) insulation, andtungsten (W) film grid and heater electrodes. This control unit can beefficiently heated, can withstand large voltages between grid andcathode, and has a high grid dissipation capacity. It is operated in thetube near 1050 degrees C.

The cathode disk used in this assembly can be an impregnated type suchas a Philips Type B or a Semicon Type S. The impregnant is removed fromthe outer surfaces prior to the BN deposition so as to prevent a directreaction with the chemical vapors. This cleaning procedure also permitsthe BN insulation to become mechanically locked in the open pores of thetungsten surface.

Chemical vapor deposition processes are used to deposit BN an W layersonto the cathode. The completed structure is made by opening holes inthe tungsten and BN layers. Other forms of the tube and of the bondedheater-cathode-grid structure are shown in U.S. Pat. Nos. 3,599,031 and3,694,260 by J. E. Beggs. These patents show the structure and themethod of manufacture, and include a discussion of alternate materialswhich may be used. The three Beggs patents are incorporated herein andmade a part hereof by reference.

In FIG. 1, the tungsten cathode 1 has open pores 2, an emissionimpregnant and an emission surface 3. An insulating layer 4 of BN isformed on all sides by chemical vapor deposition. The portion of theinsulating layer in and adjacent the lower groove is removed to providea cathode contact region 5. A tungsten film is formed over theinsulating layer, and perforations are formed by providing a mask andusing a blast gun to erode through the insulating layer to form acontrol grid 6. The tungsten film extends to the upper groove to providea grid contact region 7. A heater 8 is formed in the tungsten film onthe opposite face, with heater contact regions 9. Grid patterns withdetail as small as 0.002 inch have been formed by erosion through a maskwith air driven by Al₂ O₃ particles. U.S. Pat. No. 3,694,260 alsodiscloses forming a photo resist layer over the tungsten film,developing a grid pattern therein, forming the grid holes in thetungsten film by etching, and using the photoresist and tungsten film asa composite mask for air blast erosion of the holes in the BN insulator.

Further development of the tube structure, and method of manufacturingit have continued, to obtain a tube whose characteristics are: a peakpower output of one kilowatt at a duty factor of 0.1, a 1 db bandwidthof 400 megahertz at 3,300 megahertz, a power gain of 15 db, and anoverall efficiency of 30 percent. Calculation shows that thesecharacteristics require as tube parameters: grid-cathode capacitanceequal or less that 175 picofarads, grid transparency of 75%, insulatordielectric constant of approximately 4; cathode area equal or less that2.6 square centimeters, cathode emission density equal or greater than1.4 ampere per square centimeter average or 6.4 ampere per squarecentimeter peak.

The most important parameters for selecting the insulating film are thefilm dielectric constant, resistivity, and stability at the cathodeoperating temperature. The preferred material selected is BN. Thismaterial also has a good expansion match to tungsten, and has the uniqueproperty among high resistivity refractories of being soft and, hence,not subject to cracking due to expansion differentials.

CONSTRUCTION OF GRID PATTERNS WITH FINE DETAIL

Several means of producing grid structures with fine detail, of theorder of 1 to 0.5 mils, were attempted in this work without success.These have included air abrasion through a mechanical mask, rf sputteretching, and laser machining. Detail finer than about 2 mils is verydifficult to achieve with mechanical masks and air abrasion, because thestructural members of the mask become very weak mechanically and vibratein the air stream, allowing abrasion to occur in regions they aresupposed to mask. Radio frequency sputtering cuts through the layerswith excellent resolution even for high aspect ratio holes. However, therf sputtering is a slow process, requiring several hours. Furthermore,sputtering into the tungsten cathode deposits metal on the interior ofthe holes cut through the BN insulator, shorting grid to cathode.Several modifications to the sputtering method have been studied,including varying the d-c bias of the cathode disk, changes in themolecular weight of the gas, and use of reactive gases. None of thesemodifications improved the speed of the method sufficiently or solvedthe shorting problem.

Laser machining was not satisfactory because of the large number ofholes which must be formed one at a time, because of residual tungstendeposits on the BN insulation, and because the high transparency of thegrid requires the holes to be close together so that heat dissipationproblems arise during the laser machining. Chemical etching of BN tomake the grid structures has not been attempted because the etchingtakes are too long, the required aspect ratio of holes cannot beobtained, and severe undercutting of the structure will occur.

A process has been developed which combines photolithography, etching,and air erosion. It has been shown to provide speed in fabrication andthe requisite resolution and cutting depth. The technique shows realpromise in solving the problems of fabrication of grid patterns withfine detail.

The mechanical requirements of a bonded grid tube at the desiredfrequencies are rather stringent. The dimensions of the grid openingmust be of the order of 1 to 0.5 mils, and a cathode 3/4 inch indiameter with a grid of 75 percent transparency requires approximatelyone million holes cut through successive layers of tungsten (3 μmthick), Si₃ N₄ (1 μm), BN (10 μm), Si₃ N₄ (1 μm), and into the tungstencathode. It is permissible for a few percent of the holes to beincompletely opened. However, the layered structure cannot be allowed totear away in regions of appreciable size, because the anode will be ableto directly affect the cathode holes in those regions and grid controlwill suffer. The number of openings can be reduced to about 10,000 byusing slots having a 10/1 planar aspect ratio. After processing, thecathode surface must be clean for good emission, and the BN surfacesmust be free of conductive metal films which would short grid tocathode. There are too many holes to be economically cut sequentially,and the processing used should allow individual steps to be carried outsimultaneously on a number of cathodes with rapid setup and processing.Where processing techniques such as photolithography are used, the needfor mask alignment should be avoided.

A series of experiments were performed using air erosion with No. 400mesh alumina (ultimate particle size up to 1.5 mils). Both wire woundmasks and masks photoetched into thin metal were employed. The usualgrid structure made with wire wound masks consists of a series of longgrooves eroded through the BN insulation. As one reduces the diameter ofthe wire in the mask to make the grooves narrow and closely spaced, therigidity of the wire against bending is rapidly decreased. Masks woundfrom wire 2 mils or less in diameter did not protect the tungstensurface under the wire. The lack of rigidity of the fine wires causedthem to vibrate in the airstream from the abrasion gun. The tungstenfilm under the wires received only partial protection from abrasive andit was either abraded away or sufficiently worked and deformed byimpinging alumina to break the bond to the layer of Si₃ N₄ below.

Wire wound masks were made more rigid by the use of a structure withheavier cross supports brazed at right angles to the fine wires. Gridstructures originally designed for the L-67 tube were ideal. The finewires were then ground half round so that they would seat flat againstthe surface to be abraded. Air erosion results were identical to thoseexperienced before. The tungsten either eroded away or puckered.Improvement was obtained by gluing the mask to the cathode surface priorto abrasion. With a glued mask the tungsten under the wires did noterode and one could cut through the BN insulation. However, the wireswere not rigid enough to prevent the BN from being broken free at thelower interface near the cathode surface.

Small areas were successfully machined with a glued mask when the wirewound mask was replaced with a metal sheet into which 4 mil holes hadbeen etched on 6-mil centers. This type of mask produced a pattern withgood lateral rigidity. The tungsten was well protected and in theabsence of long, thin strips of BN insulation with poor lateralrigidity, fracture at the interface of cathode and BN was notexperienced. This method had several disadvantages, however. Gluing downof masks was difficult to achieve with good adherence in all areas and auniformly thin bond. There were difficulties in removing the maskwithout damage to the structure and contamination of the cathodesurface. The masks were not reusable.

A subsequent set of experiments was performed using photoresist as anerosion mask. The approach has the advantages of enabling one to combinechemical etching with air erosion using the same resist mask, and ofemploying an exposure mask which is not damaged and may be usedrepeatedly. Three resist systems were explored: dichromated gelatine,Waycoat SC resist (Philip A. Hunt Company), and Dynachem CMR 5000(Dynachem Corporation). Several other commercial resist materials(dichromated fish glue and dichromated polyvinyl alcohol) might beuseful, but these were not studied. Light sensitive epoxies andpolyurethanes are interesting systems to consider but were not foundavailable as resist systems. Polyurethanes in particular are reported tohave excellent erosion resistance.

The use of photo-resist masks dates from the first photoengraving byJoseph Nicephore Niepce in 1826. The use of dichromated-colloid layersas a light sensitive medium was discovered by Suckow in 1830. Thesesystems are hardened by light and chemical treatment. Patterns are madeby dissolving away the unexposed and more soluble portions of the layerwith warm water. In recent times new resist systems have been made fromlight sensitive resins that are soluble in organic solvents rather thanin water. A number of such resists are commercially available and areessential to the present chemical milling and silicon planar devicebusinesses.

Three resist systems have been explored in this work as masks forerosion lithography. The first examined was lime process gelatinesensitized with ammonium dichromate. It was selected because it wasexpected to have better abrasion resistance than the resins used incommercial resists. The characteristics of the gelatine resist led toexperiments with Waycoat SC resist. (No systematic intercomparison hasbeen made of the many commercial resist systems which are available).

The gelatine system was sensitized by adding ammonium dichromate to thegelatine solution. High concentrations of dichromate improve the lightsensitivity of the gelatine layers but lead to precipitation of ammoniumdichromate crystals during drying of the films. A standard weight ratioof ammonium dichromate to gelatine of 1:6 was found to provide goodlight sensitivity without crystal precipitation for gelatine solutionsmade up with widely varying water content and, hence, viscosity.

Resist films may be deposited by rolling, spraying, dipping, andspinning. Rolling and spraying were not used because good uniformity infilm thickness required equipment not on hand, and the immediateinterest was feasibility of resists as abrasion masks. Dipping andspinning are both reasonable processes for initial experiments. Spinningwas chosen because it produces a uniform film except at the edge of thedisk and because spinners were available in lamilar flow hoods in roomswith appropriately filtered light. Dipping coats both sides of the diskbut leaves a 10 percent taper in film thickness and excess material atthe last point to emerge from the resist solution.

Spin speeds from 2000 rpm to 10,000 rpm for 30 to 60 seconds were foundto produce satisfactory films. These speeds then set the requirements ofsolution viscosity. In order to abrade through ten microns of BN, it isnecessary to mask with a resist thickness of 2 to 7 microns. Experimentshowed that resist solution viscosity near 500 cp would result in filmsin the desired thickness range. A viscosity of 500 cp for solutions ofabout 0.31 gelatine-to-water ratio solution was prepared by stirring thegelatine into cold water to swell it and then heating gently to 40degrees C. Ammonium dichromate was added to sensitize the solution, andit was stored at 38 degrees C. to prevent it from setting. The pH of thesolution can be adjusted to control its viscosity (see S. E. Shepard andB. C. Hoack, "The Structure of Gelatine Sols and Gels, Part IV: Fluidityand Hydrolysis," Journal of Physical Chemistry, Vol. 36, 1932, p. 2319)and (see Jaromir Kosar, Light Sensitive Systems, John Wiley and Sons,Inc., New York, N.Y. 1965) characteristics, but was not adjusted fromthe value pH 5.8 obtained in preparing the solution.

Initial experiments were conducted with films spun on 1-inch-diameterquartz disks. The disk surface was flooded prior to spinning and thefilms were slowly dried after spinning in an ambient of 70 percenthumidity. A NH₄ Cl-KNO₃ solution in a desiccator jar was used tomaintain the drying conditions. A 3 -second exposure to ultravioletlight in a PRECO 200-watt exposure unit was followed by a 20-minutedevelopment with 38 degrees C. water. After development, the films werefurther hardened with ultra-violet light or mucochloric acid (see U.S.Pat. No. 2,080,019). After air abrasion was performed, the remainingresist was stripped with sodium hypochlorite or takamine gelatinate No.3 enzyme (Miles Laboratories, Inc.).

This resist system had several disadvantages. The dichromated gelatinesolution was found to be so unstable with time that viscosity increased,solubility decreased, and light sensitivity decreased. If allowed tostand at 38 degrees C. for several days the solution set. In the courseof storage for shorter periods the appropriate times for exposure anddevelopment were affected. The gelatine solution and dichromate could bestored separately, but under these conditions the viscosity of thegelatine solution decreased with time (by a factor of ten in a few days)so that film thickness was affected. Adjustment of pH improved stabilityof the gelatine resist system, but for consistent results the resistsolution needed to be freshly made. The gelatine resist is a watersoluble system. Its use is somewhat incompatible with the commercialresist systems, which are best handled in a room with low humidity. Somecomponents, such as spinners, should not be used for both resistsystems; separate facilities are therefore needed for the gelatineresist. Finally, the gelatine system was found to be subject toreticulation. Details of the order of 0.5 mil could be reproduced, butedge detail was distorted by film reticulation.

The apparent success of gelatine resist led to the examination ofWaycoat SC resist as an alternative. A comparison of erosion resistancewas made for pyrolytic BN, quartz, gelatine resist, Waycoat SC resist,and sintered tungsten cathodes. Material samples were held a standarddistance from an air abrasion gun and material removal was measuredafter eroding the sample for a period of time. The gun was operated at aconstant pressure of 80 psi and No. 400 mesh alumina was used in all ofthe measurements. The results are plotted in FIG. 2, which showsmaterial removed in mils versus the erosion time. One notes that BNerodes more rapidly than any other material examined and that quartzerodes almost as rapidly. In a number of experiments quartz disks havebeen used to model the erosion performance to be expected with CVD BNfilms. The lowest erosion rate was found for tungsten.

The relative erosion rates of tungsten and BN are so widely differentthat tungsten itself should make an excellent erosion mask. Depositedtungsten films have not been used in this manner because the impact ofthe abrasive mechanically works the tungsten, breaks its bond to lowerlayers, and makes it pucker. Once it is free from the substrate, furtherimpact can break it up. Rather, the tungsten is used as an auxiliarymask, under the patterned resist. After patterning the resist, thetungsten is chemically etched. The resist with tungsten under itcomprises the erosion mask for cutting through the BN layer. The role ofthe resist is to first protect parts of the tungsten from chemicaletching, and then serve as an erosion mask to protect the tungsten fromthe direct impact of the abrasive. The elasticity of the polymer mask isessential for distribution of the impact of the air driven particles.

The erosion of gelatine films was compared for films with and withoutammonium dichromate and UV hardening, and with and without a finesuspension of alumina in the resist solution. The best erosionresistance was found for UV hardened films and the poorest forunhardened films. The addition of fine alumina improved abrasionresistance somewhat but interfered with the resolution obtainable. Theabrasion of a Waycoat SC film, also shown in FIG. 2, is more rapid thanthat of gelatine. However, the abrasion rate of SC resist is much lessthan that of BN and the commercial resist is time stable and compatiblewith the automatic equipments and other resists in use in thesemiconductor industry.

The Waycoat SC resist was applied under conditions typical ofsemiconductor processing. Disks were prebaked for 45 minutes at 250degrees C., in nitrogen. On cooling they were coated in a highaccelaration spinner by flooding the surface with solution and spinningat 6000 rpm for one minute. Film thicknesses of 4 to 5 microns wereobtained. The spun films were allowed to sit for 15 to 30 minutes forevaporation of excess solvent and then were given a soft bake at 105degrees C. for 30 minutes. Exposures of 1 second were made in the PRECO200-watt station, which was used in the work with gelatine films. Thepatterns were developed in a PLP Model 693 spray developer with times of180-second development, 30-second KMER rinse, 30-second isoproponalrinse, and 30-second air dry. Following development, the film was givena hard bake at 150 degrees C. for 30 minutes. Resolution was checkedwith an Itek 2 progressive resolution target. In the Itek resolutionpattern, depths of the cuts are up to 8 μm. It was found to beadvantageous to operate the air abrasion gun at 30 psi and to use600-mesh alumina. Resolution of 1 to 1/2 mil has been obtained with thismasking-erosion technique; this resolution is adequate for the gridstructures which need to be fabricated.

In applying this erosion method to cathodes one can choose to chemicallyetch through the upper tungsten layer or to cut through with airabrasion. FIG. 2 shows that the erosion rate of the tungsten is muchlower than that of SC resist and that unnecessary wear of the mask wouldoccur if the upper tungsten layer were eroded. It is preferable tochemically etch through the tungsten so that the SC resist and tungstenbecomes a composite mask. The etched tungsten helps to define the holesto be abraded in the BN, and the resist above the tungsten protects itfrom mechanical working and puckering.

Initial experiments with erosion lithography for very fine detail failedtotally. In spite of the favorable erosion rates for the variousmaterials in use, the patterns would be completely destroyed in someregions and not cut at all in others. This difficulty was traced to theuse of cylindrical nozzles on the air erosion gun. To avoid penetratingthe resist it is necessary to use modest gas flow velocities for whichthe radial velocity distribution is parabolic, as shown in FIG. 3. Sincethe cutting rate is proportional to particle energy (i.e., to V²), thecutting rate varies as the fourth power of the radial distance from thecenter of the air stream. This results in the formation of a smallcentral pit.

The problem is solved by using a rectangular nozzle of high aspectratio, as shown in FIG. 4, and scanning the sample in front of thenozzle. Aspect ratio is defined as the ratio of the longer to theshorter dimension. A high aspect ratio means that the nozzle crosssection at the exit is relatively long compared to the width. Therectangular nozzles produce uniform cutting within 30 percent, over a1-inch length. Several nozzle designs were tried; the system in use wasdesigned for constant area in the transition from circular torectangular cross section.

FIG. 5 shows another double tapered nozzle having a rectangular exit ofhigh aspect ratio, designed for constant cross sectional area in thetransition from circular to rectangular cross section. The cylindricalinlet 11 has a transition 12 to a square cross section. There is avertical taper 13 and a horizontal taper 14 to a rectangular exit 15.The vertical and horizontal taper provide the constant cross sectionalarea.

FIG. 6 is a graph showing the X-Y and Y-Z velocity profiles produced bythe rectangular exit nozzle shape shown on the left, in FIG. 6A, withthe long dimension along the X axis and the short dimension along the Zaxis. The Y axis represents the velocity.

FIG. 7 shows how the nozzle 10 may be used to scan a cathode-grid unit16, shown in FIG. 7A. The nozzle 10 is mounted on a carriage 17. A leadscrew 18 through the carriage is driven by a scan drive motor 19. Theair and particle stream enters through the tube 20, and passes throughthe nozzle to the target 16 in which the grid pattern is formed. Themotion of the carriage and nozzle is such that the rectangular exit hasits short dimension in the direction of movement, and its long dimensionperpendicular thereto. It is advantageous to rotate the target 16 by 90degrees between passes of the nozzle, for more uniform abrasion.

FIGS. 8 and 9 show an alternative structure for providing a nozzle witha rectangular exit, in which the entire erosion gun housing is built inrectangular form. The nozzle 22 is completely rectangular. The air froma rectangular source 23 passes through a rectangular orifice 24, withentrance of the abrasive particles through a rectangular suction tube25.

FIGS. 11 and 12 show alterations of the exit shape from that of a simplerectangle to increase the length of the region which is uniformlyeroded, FIG. 10 rectangular exit, FIG. 11, dogbone exit, FIG. 13elliptical exit.

FABRICATION PROCEDURE FOR THE BONDED-GRID TRIODE AMPLIFIER

The bonded-grid triode amplifier is fabricated in several parallelassembly steps. The cathode-grid structure is formed as shown in FIG.13.

The cathode blanks are manufactured by Semicon Associates, Inc., asubsidiary of Varian Associates. The first step in the cathodepreparation is to polish the blanks because, as received from themanufacturer (see FIG. 14) the blanks have a lathe-cut surface. It isnecessary to dry-polish in two stages; first with a coarse-gritpolishing wheel and then with a fine polishing wheel to remove machiningmarks and 2 to 3 mils of the original surface. The blanks are thensandblasted with alumina powder to provide a rough surface for betteradhesion of the insulator layers. Residual traces of aluminum oxide areremoved by cleaning the blanks ultrasonically in ethyl alcohol. Theblanks are then hydrogen-fired at 1325 degrees C. (brightnesstemperature) for 10 minutes to remove contaminants which may have beenintroduced in the polishing operation. They are then activated in highvacuum at 1200 degrees C., to develop emission and to prepare them forthe iridium coating.

The emission capabilities of the cathodes are measured prior to iridiumcoating. Iridium is then deposited on the cathodes by a chemical vapordeposition process.

The next step in the process is to deposit the insulation on the surfaceof the iridium-coated cathodes. The insulation is a laminated structure(FIG. 13), with each discrete layer of the structure serving a specificfunction. This step of the process is again a chemical vapor deposition.

The first layer deposited is BN, 0.5 μm thick; this layer acts as astress reliever between the substrate and the subsequently depositedlayers. The next layer is Si₃ N₄ 0.4 to 0.6 μ m thick, which acts as adiffusion barrier, preventing cathode activators from diffusing into theinsulating layer. Next, a layer of BN 10 to 15 μm thick is laid down toprovide the required electrical insulation between the cathode and grid.The final layer is Si₃ N₄ 0.2 to 0.3 μm thick; this serves to improvethe adhesion between the metallic grid film and the insulatingstructure.

The grid film coating step follows the insulation coating. The metallicgrid film is also obtained by a chemical vapor deposition process. Inthis case molybdenum carbonyl is decomposed on the cathode surface. Thetemperature of the cathode is held at 1075 degrees C. A partial pressureof hydrogen is used to prevent carbide formation. The thickness of thefilm is about 5 μm, obtained in a 45-minute coating cycle. The hydrogenpressure is about 20 microns; the Mo(CO)₆ +CO is also about 20 microns.

The grid and heater structures are photolithographed according to thefollowing steps:

1. Application of photo-resist. The photo-resist material is spread overthe surface of the cathode by means of a fresh, eye dropper type ofdropping pipet. The cathode is then rotated at high speed (2000 to 8000rpm). This spreads the photo-resist material into a thin, uniform layer.

2. A short baking cycle follows, during which the photo-resist layer isdried.

3. The process is then repeated on the opposite face of the cathode.This coat is also dried.

4. The grid and heater patterns are then formed by exposing theappropriate faces of the cathode through a mask to form the requiredpatterns in the photo-resist.

5. Each unit is next put through a developing process which removes theunexposed photo-resist.

6. The final step in the photolithographic procedure is a bake whichcures the photoresist and gives it the required toughness.

The grid detail is then developed in the following steps:

1. The metal film is removed from the grid openings using an acidchemical etch having the composition:

76 parts by volume H₃ PO₄ (phospheric acid)

6 parts by volume CH₃ COOH (acetic acid)

3 parts by volume HNO₃ (nitric acid)

15 parts by volume H₂ O (water, distilled)

The etch time is 9 to 15 minutes. The heater side is etched at the sametime to remove extraneous metal and leave the metal film heater pattern.

2. Nitride insulation is removed from the grid openings by the airabrasion method, using air-classified Al₂ O₃ powder from which the fineand coarse fractions have been removed. A specially designed nozzlecoupled to an automatic scanning device, with controlled air pressure,provides uniform abrasion over the entire exposed insulator surface ofthe cathode. The photoresist was previously developed to a toughnessthat will withstand the air abrasion until the insulation issubstantially removed from the grid openings.

3. The cathode is subjected to ultrasonic cleaning in ethanol to removeAl₂ O₃ particles which might be imbedded in the cathode surface.

4. The photo-resist is removed by heating the cathode to approximately400 degrees C. in a low-pressure (10 microns) hydrogen atmosphere. Atthis temperature the photo-resist evaporates leaving no residue.

5. The cathode is again subjected to ultrasonic cleaning in ethanol toremove Al₂ O₃ particles which had been imbedded in the photo-resist andstill remain.

6. Any insulation remaining in the grid openings or lodged in the poresof the cathode is removed by etching with ionized freon gas.

7. The final step is firing the unit in hydrogen to remove surfacecontaminants and aid in reactivation of the cathode. This step ensurescomplete removal of fluorides. The structure is now ready for mountingwithin the vacuum enclosure.

FIG. 15 is a drawing made from a photomicrograph of a grid structuremade by erosion lithography. The openings are 1.2 mils by 8.2 mils. Thedepth is 0.75 mils, the transparency 47%, and the density 48,000 holesper square inch. The machining time for a 3/4 inch diameter structurewas four minutes. The minimum hole size by this process is 1/2 mil. Themaximum density of holes by this process is 1,000,000 holes per squareinch. Transparency is defined as the ratio of the area of the openingsto the total area of the surface in which the pattern is formed.

CONCLUSION

To summarize the above, air erosion with abrasive powders is a rapid andefficient method of removing material. Application of the method hasbeen limited in many instances by ability to erode uniformly over a widearea. This limitation arises from the very rapid variation of cuttingrate with radial distance in the cylindrical air stream generallyemployed. Air velocity varies approximately quadratically with radius inthe cylindrical air streams, and cutting rate varies between particlevelocity to the 2nd power and to the 2.3 power. In consequence, cuttingrate with a cylindrical air stream varies at least as the fourth powerof radius. Even a large diameter air erosion nozzle cuts a work piece inonly a small region near the center of the air stream.

The solution is a nozzle with a noncylindrical cross section of largeaspect ratio to provide uniform erosion over a wide region. Uniformerosion of a large area of a work piece can be accomplished by scanningthe work piece below the nozzle. Several designs are useful. All useexits with a large aspect ratio; the simplest being a rectangularopening.

What is claimed is:
 1. A bonded grid-cathode structure comprising a hotthermionic cathode planar structure for operating temperature greaterthan 1000° C., covered by an insulating layer of boron nitride and thatlayer covered by a grid layer of molybdenum or tungsten refractorymetal, there being identical, aligned openings completely through bothgrid and insulating layers, the openings having rectangular, ellipticalor dog-boned top shape, width of grid openings substantially smallerthan two mils, planar aspect ratio of openings at least 10/1length-to-width, and having grid transparency of at least approximately70 percent formed by the method of making a bonded grid on a cathode,comprising the steps of:(a) forming an insulator on said cathode, (b)forming a metal layer for the grid, on the insulating layer, (c) forminga photo-resist layer on the metal layer, (d) exposing the photo-resistlayer with means to form a grid pattern, followed by a developingprocess which selectively removes part of the photo-resist layer toexpose openings of said grid pattern, (e) removing the metal layer insaid openings, (f) using air blasting with an abrasive powder to removethe insulator layer in said openings, with the photo-resist on the metallayer acting as a mask, wherein a special nozzle and scanning providessubstantially uniform abrasion over the entire exposed surface of saidinsulator layer.
 2. The bonded grid-cathode structure of claim 1 inwhich the width of the grid openings is substantially smaller than onemil.
 3. The bonded grid-cathode structure of claim 1 in which the widthof the grid openings is substantially smaller than one half to one mil.4. A bonded grid-cathode structure comprising a thermionic cathode, ofmolybdenum material, of planar surface covered by an insulating layer ofboron nitride and that layer then covered with grid layer made of arefractory metal, there being identical, aligned grid openings ofdog-boned shaped made completely through both grid and insulating layerswhere transparency is at least 70 percent and width of grid openings isnot greater than two mils formed by the method of making a bonded gridon a cathode, comprising the steps of:(a) forming an insulator on saidcathode, (b) forming a metal layer for the grid, on the insulatinglayer, (c) forming a photo-resist layer on the metal layer, (d) exposingthe photo-resist layer with means to form a grid pattern, followed by adeveloping process which selectively removes part of the photo-resistlayer to expose openings of said grid pattern, (e) removing the metallayer in said openings, (f) using air blasting with an abrasive powderto remove the insulator layer in said openings, with the photo-resist onthe metal layer acting as a mask, wherein a special nozzle and scanningprovides substantially uniform abrasion over the entire exposed surfaceof said insulator layer.
 5. The bonded grid-cathode structure as inclaim 4 wherein the grid comprises tungsten.
 6. The bonded grid-cathodestructure of claim 5 wherein the openings have a rectangular orelliptical shape.
 7. The bonded grid-cathode structure of claim 6wherein the openings have planar aspect ratio of at least 10/1.
 8. Thebonded grid-cathode structure of claim 5 wherein the width of the gridopenings is less than one mil.
 9. The bonded grid-cathode structure ofclaim 5 wherein the width of the grid opening is less than one half mil.